So I had an idea,

The 'play now' button can never put you on a password protected server.

So password protect your server, and put the password in the server title.
That way you can hint at the Elo level you want if you wish, and either way only people actually using the server browser and knowing to look under the private listings as well will be able to join.

It's better than running invite only or just plain passworded as it does mean you have a much larger potential player base than just your own friends.

It's not ideal, but maybe it could help if it catches on (as if good players don't pass the idea on then people won't be looking at the private server list)

I've started "10rnd, pass: 2k-elo" on .DE if anyone wants to use it.